1. Field of the Invention
The present invention relates generally to raise type earth boring drills and, more particularly, to such raise drills having shock elements for absorbing impact type loads passing through the drill.
2. Description of the Prior Art
A relatively large diameter hole may be provided between two locations in a mine by an operation commonly referred to as raise drilling. A raise drilling operation begins by drilling a small diameter pilot hole through the earth between the locations using a small diameter pilot bit. After the pilot hole is completed, the pilot bit is removed from the drill column and a large diameter raise bit is attached. The raise bit is then rotated and drawn along the pilot hole to enable the drill cutters to contact and disintegrate the earth formations surrounding the pilot hole, thereby enlarging the pilot hole to the desired size. In an exemplary embodiment, the pilot hole may be 11 inches in diameter and the reamed out hole may be six feet in diameter.
During a raise drilling operation, a tremendous amount of wear and stress is imposed upon the raise bit. The drive stem in particular is subjected to considerable wear due to abrasive contact with the surrounding earth formation and is also subject to considerable stress resulting from (a) tension due to the pulling force imparted to the drill, (b) twisting due to the torque applied to the drill, and (c) bending due to uneven loading around the circumference of the drill. Because of this considerable wear, many raise drills have the drive stems replaceable mounted on the bit body, thereby extending the useful life of the bit. The low profile of the separated components allows the raise bit to be transported through small drifts or passages.
The disadvantage of the replaceable drive stem is that a certain amount of down time is still required to remove and replace the stem. This non-operating time is costly and it is still preferable to obtain as long a running time as possible for each bit-stem combination.
As a result other features have been incorporated into the raise drill which have prolonged the operating life of such drills. One such feature has been the addition of an elastomeric shock element for absorbing the impact type loads passing through the drill. One such shock absorbing system is described in Assignee's copending application Ser. No. 833,040, filed Sept. 14, 1977. However, with the utilization of such a component, the drill bit body is relatively movable with respect to the drive stem. As a result, the inboard cutters, which are normally connected and anchored to the drive stem for stabilization and load bearing purposes, cannot now be anchored to the drive stem and still have the bit body flexible mounted.